Polarity is a common phenomenon in most organisms. Its simplest case is the uneven distribution of plasmatic components within cells. The shift of the nucleus towards one pole has already a chief influence on the unequal division of the cell that renders two daughter cells of different size. The development of special organelles like a flagellum determines a clear anterior and posterior pole.
Polarity does often reflect the ontogenesis of a single cell as we will see later on in the case of unicellular algae, yet the same phenomenon occurs in higher multicellular plants. We discussed the topic already in the context of the information contained in a cell's position within a tissue. Polarity may, too, be induced by extern factors like light. It is usually irreversible and remains after the extern stimulus has been removed. The maintenance of a polarity that extends over several cells makes it necessary that these cells exchange information. Cells are usually interconnected via plasmodesmata. Their plasma forms a continuum or symplast. The transport of substances from cell to cell via plasmodesmata is called symplastic transport. Water-soluble, low-molecular substances can also be transported apoplastically (apoplastic transport). In nearly all cases is the cell wall a net of such large mashes that it puts up no resistance against the flow of water and its solutes. Symplastic and apoplastic transport work only over short distances (a few cells) since their velocity is determined by diffusion.
A gradient that extends over several cells requires a continuously active local source of the gradient-forming substance (also called inductor) and an outlet (a sink) at the other end of the cells that keeps the concentration of the inductor low.
Such sink-source concepts are discussed in the context of plant hormones. They suffer from the fact that gradients established like this are extremely delicate since many possibilities to influence the inductor concentration of single cells exist. A cell cannot determine whether a low concentration of inductor is caused by its low production or by losses of the inductor on the way.
W. WERNICKE and L. MILKOVITZ (Australian National University, Canberra) detected in 1984 the existence of gradients in tissues of different developmental stages. They cultivated callus cultures of cells taken from still growing wheat leaves in the presence of the growth-regulating substance 2,4-D. It showed that the cells of the zone next to the belt of vegetation and that of the belt of vegetation itself were most sensitive to 2,4-D, i.e. they were able to regenerate to shoots and roots. In contrast could most cultures of non-meristematic leaf cells not be induced to develop shoots though they kept their ability to develop roots. The closer the cells had been to the belt of vegetation the higher was the probability that they developed shoots and roots.
In the case of the wheat leaves was it experimentally ascertained that the extend of the gradient's steepness is genetically fixed. It varies in six of the examined wheat varieties (part species, part varieties). The decrease of sensitivity of the cells towards 2,4-D has two aspects: it explains on one hand at least partially the difficulty to regenerate whole plants from callus cultures or protoplasts of monocotyledons. On the other hand clarifies the experiment the selective herbicide effect of 2,4-D on dicotyledons. Dicots seem not to lose their sensitivity towards this growth-regulating substance. Their tissues start to grow unregulated if exposed to 2,4-D, a fact that finally causes the death of the plant.
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de